1. Field of the Invention
This invention relates to a developing device. More particularly, it is concerned with a dry-type developing device using a two component developer consisting of toner and carrier for developing an electrostatic latent image formed by the electrophotographic process, etc.
2. Description of Prior Art
For high speed development and low potential development of an electrostatic latent image having a low dark potential, a developing device using a two component developer consisting of carrier and toner is suitable. In such developing device, since the developer is conveyed at a high speed, it is necessary that the developer be uniformly distributed in the developing device, and that the developer be sufficiently agitated to render a mixing ratio between the carrier and the toner in the developer to be constantly uniform. In particular, the developing device using an automatic toner replenisher (ATR) of a volume detection type for maintaining constant the mixing ratio between the carrier and the toner requires sufficient agitation for the uniform mixture.
FIG. 1 of the accompanying drawing shows, in cross-section, one embodiment of the developing device suitable for such high speed, low potential development. This developing device is provided with a conveying sleeve 1 to take up the developer, and developing sleeves 2A, 2B to effect the development. These sleeves 1, 2A, and 2B rotate in their respective arrowed directions, a, b and c, and have within their interior fixed magnet rollers 3, 4A and 4B, respectively.
Agitation of the developer in the axial direction of the sleeves is effected by a pair of screw means 5A, 5B with their conveying directions mutually opposed. The toner from a hopper and the toner recovered by a cleaner are supplied into the screw means through a front side as viewed from the top surface of the drawing. In the drawing, the screw means 5A is for toner forwarding, and the screw means 5B is for toner returning. These screw means 5A, 5B are positioned in a developer sump 7 at the downstream region of a developer recovering path 6. They function to agitate the developer within the toner sump 7 and simultaneously cause the developer to circulate in the axial direction of the screw. A doctor blade 8 in the developer sump 7 regulates a quantity of the developer adhered on the sleeve 1. The developer as adhered on the conveying sleeve 1 is sequentially transferred to the developing sleeves 2A and 2B to develop the latent image on a latent image holding member 9. The developer, which has passed through developing positions n and s for development of the latent image, leaves the sleeve 2B on its own dead weight and falls into the developer sump 7.
Even in high speed development using the developing device of the above-described construction, if its speed is relatively slow, the uniform distribution of the developer and the uniform mixing ratio between the carrier and the toner in the developer within the developing device are well maintained. However, when the speed becomes very high, a balance is lost between a quantity of the developer which the sleeves convey and a quantity of the developer which the screws means convey. On account of this, the developer in the developer sump 7 cannot be uniformly distributed on the conveying sleeve, whereby erection of ears of the developer becomes non-uniform, and, even if the developer becomes uniformly distributed temporarily, its maintenance and stabilization are very difficult. This is considered to be due to the developer of low toner concentration, which has fallen from the sleeve 2B after the development, being directly attracted to the conveying sleeve before it is sufficiently agitated in the developer sump.
FIG. 2 of the accompanying drawing shows, in cross-section, another embodiment of the developing device as disclosed in U.S. Pat. No. 4,177,757. In the drawing, those parts having the same functions as those of the developing device shown in FIG. 1 are designated by the same reference symbols.
In this developing device, the developer on the sleeve 2B is scraped off at the forward edge of a partitioning plate 10 to cause substantially the entire part of the developer on the sleeve 2B to fall into the developer sump 7, and to return the same to the sleeve 1 after the developer is conveyed in the axial direction of the sleeves by the screw means 5B along with its agitation.
On the other hand, a small quantity of the developer falling from the sleeve 2B is led to a developer storage chamber 11. This developer storage chamber 11 temporarily stores therein the developer which will be circulated in the developing device. By keeping off the developer, for a while, from its high speed moving to recuperate its function, it is possible to gradually replenish the recuperated developer through an opening 15 into the circulation path, whereby the life of the developer in the developing device as a whole can be prolonged. By rotation of blades 12, 14 at a very slow speed through a feeder shaft 13, the developer is pushed out into the developer sump 7 bit by bit through the opening 15 of the storage chamber 11. Then, the replenishing developer is fed into the storage chamber 11 from the forward end of the buffer plate 10 for an amount pushed out of the opening 15.
In the above-described developing device, as compared with the device shown in FIG. 1, the life of the developer becomes prolonged because it is temporarily stored in the developer storage chamber 11 for recuperation from its fatigue caused by its repeated use. On the other hand, however, the sleeve 1 continues to attract and convey the developer from the developer sump 7, on account of which the problem as stated in reference to the device shown in FIG. 1 remains unsolved.